March 1, 2010

Competitive Maintenance Strategies

By Dr. Robert Peltier, PE

Many consultants are prospering today by creating "new" maintenance strategies. What they’re really creating is new buzzwords. At a recent conference, for example, speakers debated the pros and cons of reliability centered maintenance, profit-centered maintenance, equipment-based maintenance, total productive maintenance, and proactive maintenance. But though the titles were slightly different, the overall concepts were amazingly similar: Competitive maintenance, the experts all seemed to agree, uses an analytical process and condition-monitoring tools to select the optimum maintenance strategy for each plant system. Note that the optimum strategy for the plant’s single steam turbine will be different than that for its four chemical-injection pumps. When the dust settles from all the discussions, there are only three ways to maintain a piece of equipment:

• Corrective maintenance. This is often called "crisis management" or "run-to-failure," because maintenance or equipment repairs are scheduled only when there is a noticeable deterioration in machine condition — like the shaft just separated from the coupling and went flying through the air. Corrective maintenance is marked by a high percentage of unplanned maintenance activities, high replacement-part inventories, and inefficient utilization of maintenance personnel, but it is still the right choice for certain pieces of equipment.

• Preventive maintenance. Often called "time-based maintenance," preventive maintenance (PM) represents a shift away from unplanned to planned maintenance activities. Periodic inspection and maintenance are scheduled at prescribed intervals in an attempt to reduce or eliminate equipment failures. Depending on the intervals set, this can represent a significant increase in inspections and routine maintenance; however, it should also reduce the frequency and seriousness of unplanned machine failures.

• Predictive maintenance. Also called "condition-based" maintenance, predictive maintenance (PdM) replaces the arbitrary time intervals with maintenance scheduled only when the condition of the equipment warrants it. The increase in cost of equipment monitoring should be more than offset by a reduction in unnecessary PM inspections and unplanned machine failures. But it takes time and money to properly perform PdM maintenance, and not every piece of equipment is worth that investment.

Unlike these three methods of maintenance, reliability-centered maintenance (RCM) is a process — a process by which the appropriate method, or combination of methods, is selected. RCM is a highly organized, statistical approach to equipment maintenance. Maintenance activities are determined for a specific machine or machine type based on the goal of having it operate in the most reliable way possible at the lowest cost. For example, a chemical-injection pump might be run to failure because it is not critical to production and cost-effective simply to replace; a chain drive may be placed on a PM program with daily lubrication; and a steam turbine will be monitored with sophisticated diagnostic tools.

Another Link in the RCM Chain

Failure mode and effects analysis (FMEA) is a key step in implementing RCM. It sounds complicated — which is what the consultants want you to think — but it’s simply a tool to quantify what you know and assemble it in a logical manner. Perhaps without realizing it, you’ve done many FMEAs whenever you had a plant incident. You analyzed why something failed, looked for the root causes, and made the changes necessary to prevent it from happening again — or at least minimize the chances. An FMEA walks through the same exercise, using hypothetical failures of plant functions, which then allows you to make smarter maintenance plans.

The table included here can be used to perform FMEA. As you follow across the table, from system to failure modes and causes to failure effects, you understand the relative importance of each plant system or piece of equipment. This understanding helps you decide how much preventive and predictive maintenance each system warrants. When developing the FMEA, don’t get too imaginative with possible failure modes; consider only those that have a high probability of occurrence.